Fenders like that have expanding plugs in them which push up into your fork's steerer tube and expand to hold it in place. If you don't have an opening in the bottom of your fork then you have to find a different product. Depending on the particular fork, there are a few options which may be able to work for you. I'd start with the type I posted above, they are the easiest to work with.
Don't you hate it when a sentence doesn't end the way you think it octopus?
There's v-brake plates they aren't made for the purpose you want one for but they'd work.
They attach to the fork legs/seat stays with hose clamps and they have a center hole where you could attach the mudguard, the brake posts on them can be unscrewed.
Someone on the commuting forum had a similar problem. He solved it with a long piece of allthread that dropped down from a star nut near the bottom of the steer tube. Not as elegant as some solutions, but another choice at least.
"Don't buy upgrades; ride up grades." -Eddy Merckx